1. Field of the Invention
The present invention relates to fasteners in general, and more particularly to a banding system for use in securely attaching strakes and fairings onto cylindrical structures used in the offshore industry.
2. Description of the Related Art
Vortex-induced vibrations (VIVs) are a concern in the offshore industry where elongated structures, such as tension leg platforms, spars, etc., are utilized. VIVs may be generated when fluid flows past an elongated structure. As current flows past a structure, friction at the structure/fluid interface slows proximal fluid flow. Vortex eddies result from the differential fluid velocity of the proximal and distal fluid flow.
Floating structures possess long, cylindrical components, such as steel catenary risers (SCRs), tendons, cables, flowlines, pipelines, among others, that are sensitive to VIVs resulting from ocean currents. VIVs have been shown to greatly reduce the life span of cylindrical structures.
Vortex-induced vibration suppression devices have been developed in order to combat VIVs. Vortex strakes consist of a generally elongated cylindrical shell that typically contains three fins running longitudinally along the outside surface. These fins define a three-start helical pattern that spirals around the shell in a set period or frequency. The primary function of the fins is to prevent formation of vortex eddies by disrupting the current flow. Vortex strakes are typically firmly attached to the cylindrical structure in need of protection and are not permitted to rotate.
Fairings are typically a tear-dropped or U-shaped device that function by weathervaning along with the current flow in order to disrupt the current flow and prevent the formation of vortex eddies. The fairings encapsulate the cylindrical structure to be protected. The weathervane function of the fairing is essential. For this reason the fairings are not snuggly affixed to the cylindrical structure. Thus a thrust collar is required to maintain the fairings elevation. The thrust collar is fixedly attached to the cylindrical structure to prevent the fairings from unintentionally migrating along the length of the cylindrical structure.
A majority of the cylindrical structures, i.e., SCRs, flowlines, pipelines, etc., are coated with a thermally insulating material to decrease the resistance to flow typically encountered due to the frigidness of the working environment. Typically these cylindrical structures are required to function at water temperatures as low as 1.67° C. to 4.44° C. (35° F.-40° F.). The insulation is utilized to prevent heat loss from the inside of the cylindrical structures in order to keep the production material flowing at higher rates and to prevent freezing or the formation of hydrates as the case may be.
Several different types of insulation materials are utilized in the deep-water offshore oil and gas industry. The most common material is a glass-sphere syntactic epoxy foam (GSE), this is an epoxy resin that is blended with hollow glass spheres containing air to increase their insulating properties. Although epoxies provide excellent insulating characteristics, they are also stiff, brittle and inflexible. GSEs have fractured, cracked and broken away from the cylindrical structures they were insulating due to their brittleness. These occurrences permit desirable heat to escape from the cylindrical structures contents typically causing a slow down in production.
Glass-sphere syntactic polyurethane insulation (GSPU) has risen in popularity due to its flexibility, which in turn resists cracking or breaking while utilizing current pipeline laying techniques. However, as a natural byproduct of this desired increase in flexibility the naturally occurring hydrostatic pressure exerted by the water, that has no discernible effect on the stiff, inflexible epoxy resin, actually compresses the GSPU at great depths causing a decrease in the outer diameter. Vortex strakes or fairings are typically installed onto the thermally insulated cylindrical structures on the deck of pipe-laying barges prior to deployment. Typically, this attachment is achieved with the use of widely used corrosion resistant banding material. Although this procedure is sufficient when GSE is used as the insulator, it is impractical when utilizing the more desirable GSPU insulation.
Once these typical fasteners are used and the cylindrical structure is placed into the water, the hydrostatic pressure will begin to affect the outer diameter of the cylindrical structure. The bands, which were correctly tightened at the ocean's surface, become loose due to the ensuing hydrostatic pressure at lower depths. The strakes are then free to rotate along with the current, slide along the cylindrical structure, and/or become inter-tangled with other strakes, all of which are not desirable. Hydrostatic pressure also affects the thrust collars of fairings installed over GSPU insulation allowing the fairings to move along the length of the cylindrical structure away from the area where the fairings are needed.
During a typical installation, risers can be, and are usually, placed on the ocean floor where they typically remain for months at a time prior to their final installation. Risers are subjected to cold-water temperatures and hydrostatic pressure that results in compression of the insulation material. Typical banding material is ill-equipped to handle this resultant compression. Once the risers are raised for installation, typical bands will allow the strakes and/or fairings to undesirably travel along the riser due to their resultant slackness, allowing the strakes and/or fairing to move from the desired position. Further, once the risers are correctly positioned and production begins, the risers tend to heat and expand from their original cold position. Conventional bands are as inflexible with outward extension as they are with compression and are not able to fully function in these standard installation and production conditions.
Flaws in mainstream bands are also apparent during hurricane conditions when wells are typically shut down. The shut down effectively halts production and clears the lines of flowing product. Once this shut down occurs the affected pipelines tend to cool causing compression of the insulation. Again, standard bands cannot compensate for the resultant changes in diameter that occur due to this compression nor with the resultant expansion that will occur once the well is reinitialized. These shortcomings result in the movement, or further movement, of the attached strakes and/or fairings from a desired position.
It would be an improvement to the art to have a compliant banding system that is able to maintain sufficient tension at different depths in order to securely hold a vortex strake or a fairing collar to a cylindrical structure when hydrostatic pressure is affecting the outer diameter of the structure.
It would be a further improvement in the art to have a compliant banding system that is able to maintain sufficient tension without overstressing the straps should there be an increase in temperature inside the cylindrical structure causing the outer diameter to increase.
It would be a further improvement in the art to reduce the time required to install vortex-induced vibration inhibitors on cylindrical structures.
It would be a further improvement in the art to allow for competitive pricing when attempting to secure projects in the offshore oil and gas industry.